The present invention relates to a power supply system for driving an electric rotating machine, particularly to a power supply system for driving an electric rotating machine suited for vehicle use.
FIG. 10 is a schematic view showing a first example of the arrangement of a related power supply system for driving an electric rotating machine provided in a driving system of a hybrid vehicle. The power supply system is disclosed in the following non-patent document: H. Hanada et al., TOYOTA Technical Review Vol. 54, No. 1, August 2005, pp. 42-51.
The first example is presented as a power supply system in a driving system of a hybrid vehicle provided with both of an internal combustion engine and a motor as driving power sources of a vehicle. The driving system is made up of components such as an engine ENG, gears G, a driving power split mechanism PG, a generator MG1, a motor MG2, inverters INV1 and INV2, a battery voltage step-up converter CHP, a battery BT and wheels T.
In the power supply system shown in FIG. 10, three-phase alternating-current, generated by the generator MG1 driven by the engine ENG through the power split mechanism PG, is once converted to a direct-current by the inverter INV1. The inverter INV2 further converts the converted direct-current to a three-phase alternating-current with an arbitrary voltage and a frequency, which drives the motor MG2 to thereby drive the wheels. Moreover, electric power for driving the wheels T is also supplied from the battery BT. Thus, the battery voltage is stepped-up by the battery voltage step-up converter CHP.
With the above procedure, in addition to the electric power supply from the generator MG1, electric power can be supplied from the battery BT. This can reduce an electric capacity of each of components for the electric power supply to enable downsizing of the system. Furthermore, with the direct-current voltage, being stepped-up by the battery voltage step-up converter CHP, adjusted at a voltage with an arbitrary value, the whole electric energy transmitting section can be made highly efficient. In the following, the electric energy transmitting section will be simply referred to as the system.
FIG. 11 is a schematic view showing a second example of the arrangement of a related power supply system for driving an electric rotating machine. The power supply system is disclosed in JP-A-2005-318731 (corresponding international publication number: WO 2005/105511 A1) and JP-A-2006-340470. This is also an example of the power supply system provided in the driving system of a hybrid vehicle. The differences in the second example shown in FIG. 11 from the first example shown in FIG. 10 are that (1) the inverter INV1 in the first example is substituted by a matrix converter MC and (2) while the direct-current output section of the inverter INV 1 was connected to the voltage step-up section of the battery voltage step-up converter CHP in the first example of the related power supply system shown in FIG. 10, in the second example of the related power supply system shown in FIG. 11, the three-phase alternating-current output section of the matrix converter MC is connected in parallel to the three-phase alternating-current output of the INV 2. In this case, the electric power generated by the generator MG1 is directly supplied to the motor MG2 without the intervention of the inverter INV2.
The matrix converter is well-known as a circuit for directly converting a three-phase alternating current from a power supply to a three-phase alternating-current with an arbitrary voltage and frequency by operating bi-directional switches to carry out pulse width modulation (PWM). Thus, the explanation thereof will be omitted.
In the configuration of the power supply system shown in FIG. 10, the generator MG1, the motor MG2 and the battery BT are connected to one another with a main capacitor C with their voltages converted by the inverter INV1, the inverter INV2 and the battery voltage step-up converter CHP, respectively. The voltage applied to the main capacitor C is made variable according to the operating conditions of the generator MG1 and the motor MG2 to thereby reduce a system loss for enhancing the motor efficiency.
In the arrangement in the power supply system shown in FIG. 10, however, when the power generated by the generator MG1 is converted to the power supplied to the motor MG2, an electric current is fed to the two inverters INV1 and INV2 through such a path as that of the inverter INV1→the main capacitor C→the inverter INV2. This causes a problem of increasing losses produced in the inverters.
While in the arrangement in the power supply system shown in FIG. 11, for eliminating the drawback in the power supply system shown in FIG. 10 to improve the system, direct conversion of the electric power is carried out by the use of the matrix converter MC to reduce the losses produced in the inverters. Moreover, an output of the battery BT is converted to a three-phase alternating current with the inverter INV2 via the battery voltage step-up converter CHP. The three-phase alternating-current output of the INV2 is connected to the motor MG2 in parallel to the three-phase alternating-current output of the matrix converter MC.
Both of the matrix converter MC and the inverter INV2 function as three-phase alternating-current power supplies for the motor MG2. Therefore, when a slight potential difference is caused between the output of the matrix converter MC and the output of the inverter INV2, the output current of the matrix converter MC and the output current of the inverter INV2 becomes unbalanced. This makes it practically impossible to simultaneously use both of the matrix converter MC and the inverter INV2.
A countermeasure against this is to insert an impedance element such as a reactor between the output terminals of the matrix converter MC and the inverter INV2. This makes it possible to balance the output currents of the two. However, the matrix converter MC and the inverter INV2 are connected in parallel to each other and the matrix converter MC can not step-up the voltage generated by the generator MG1. This, as far as the electrical specifications of the motor MG2 and the battery BT are the same as those in the power supply system shown in FIG. 10, causes problems of making it impossible to raise the voltage for reducing the system loss and enhancing the motor output as was described about the system shown in FIG. 10.
Accordingly, it would be desirable to provide a system to reduce an electrical loss and to enhance a motor output even though a matrix converter is used in the system.